Method of preparing photographic emulsions



Patented Oct. 21', 1952 UNITED STATES PATENT- 2,614,928 METHOD oF PREPARING PHo'roGRAPHIof r 2,614,928 *Q FI E EMULSIONS Henry C. Yutzy and Gordon F. Frame, Rochester; N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application August 13; 1947,

Serial No. 768,475 x 16 Claims.

This invention relates to the preparation of photographic emulsions having silver halide as the sensitive element in which the silver halide is prepared in dispersed form in a gelatin derivative. The gelatin derivative-silver halide dis persion is coagulated and washed. This, invention also includes the products obtained thereby washed silver-halide grains u ng gelatin or gela tin derivative as the vehicle erefor.

The most common'method of preparing silverhalide dispersions is by reacting a water-soluble and the emulsions obtained by re-dispersing the f salt, such as silver nitrate and a water-solub1e halide, such as potassium bromide in an aqueous solution of gelatin. There is formed thereby a' dispersion of the silver halide in an aqueous solu-- tion of gelatin, which solution also contains salts resulting as a by-product of the. reaction. In processes of making silver-halide dispersions and emulsions using the natural productgelatin the A emulsion maker hasin the past been limited in the scope of his operations by the physical properties peculiar to that material. Thesephysical properties have necessitated the use of concentrations of silver halide and. of gelatin in photographic emulsion formulae WhlCh are not the most desirable for all pb atographic products. Forinstanoe, emulsions ofhi'gh concentration of silver halide have been clifficult to prepare; Limitations have been imposed on the emulsion maker 5 soluble salts present in the emulsion be reduced in concentration. It has been the practice previously where gelatin has been used as the protective colloid to remove the soluble materials therefrom by setting the gel dispersion, noodling, and

Washing the noodles by osmosis in cold water. Under such conditions a minimum gelatin concentration of approximately 4% is useful under practical conditions and this limitation has hampered the operations of the emulsion maker in his preparation of emulsions by this method.

One of the objects of our invention is to prepare dispersions of silver halides from which water-soluble by-products may be readily removed. Another object of our invention is to provide a convenient and practical method of preparing silver-halide dispersions in which the dispersing colloid need be but a small percentage of the total composition. A further object of our invention is to provide a method of washing silver-halide dispersions in which substantially all of the liquid may be removed, thus making for complete removaloi the water-soluble impurities therein. A still further-object of our invention is toprovide a methodof preparing silver-halide dispersions in dry form wherebyfthose dispersions can be readily transported, in a compact conchtion. Other-objects of our invention will appear Our invention is based upon the discovery that gelatin derivatives are useful as peptizers for dispersions of silver halide and that those derivatives are acidcoagulable and will envelop the silver grains when so precipitated. The method of our invention avoids the limitations which have previously characterized the preparation of silver-halide dispersions and is valuable, for exvhanding thepossible(conditions for the manufacture of photo raphic silver halide emulsions of difierent speeds, contrast, andother photographic characteristics. In addition, our invention offers a simple meansof preparing emulsions of high silver concentration,' this being especially valu able in the manufacture of photographic films bearing large quantities of silver per unitarea.

Gelatin derivatives areprepared byreacting.

, gelatin with a reagent, such as asulfonyl chloride, a carboxylic acid chloride, a, carboxylic acid anhydride, an aromatic isocyanate, or a 1,4 diketone at a pH of8-1l or more imparted by the addition of asuitable base, such 'as sodiumhydroxide or ammonium hydroxide, preferably at a temperature of 2060 C. Thereaction between I the gelatin and the reagent employed therewith takes place within a very few minutes, and, the

resulting product may be employeddirectly i'nour invention without separating it from the solution employed as dispersing agents for silver halide have the property of enveloping the silver-halide grains upon acidulation of the mass and that the resulting insoluble material consisting of silverhalide grains enveloped by the gelatin derivatives may. be washed free of salts or any other watersoluble materials which may be present therein. To effect coagulation it is desirable to acidify the mass to a pH of approximately 4.5 or less. When inert salts are present at relatively high concentration, the per range for this acid coagulation phenomenon may extend as low as 3. If, however, the salt concentration is less, the pH range for the acid coagulation is less broad, depending preparing the silver halide in an aqueous solution of the gelatin derivative,'or the silver halide may be prepared in an aqueous solution of gelatin as the peptizer, and the gelatin thus present may .changes'of .water. .--method,1,the coagulum may be redispersedin be converted into the derivative prior toacidifying and washing. Where the gelatin derivative is employed as the protective colloid that material is employed in substantially the same quantity as that of the gelatin usually used in'preparing silver halide emulsions. Also, the silver halide maybe prepared by any of the normal methods for emulsification of the silver halides 'for examplegby introducing a stream of an aqueous-solution-of silver nitrate and a stream of an aqueous. solution:

of an alkali metal halide, such as potassium-bromide, into a vigorously agitated aqueous. solution of the gelatin derivative. After the silver halide hasbeen formed andafter an ripening operation .which may be, considered advisable, acid is. added ..in.-.a..sufficient. amount to lower the pH: to, less ..than. 4.5, the; range. of pH depending ,uponthe amountof salt present as explainedabove. The

..amount,- of acid .is such. that the, gelatin .de'rivative becomes insoluble inthe reaction mixture and coagulatesand settles. carrying with it the silverhalide grainswhich .arepresent. I. The coagulum may. now be. separated. from, the liquid by any one .ofseveral. techniques. In the. simplest form this rsupernatant motherv liquor may be removed'from .the receptacle .in which. the .mass .is. contained by A means .of..a siphon and the, coagulum may be rinsed with water to remove excess insoluble salts. .Thecoagulum after this. washing operation :consists ..of, silver halide, the gelatin derivative, .any. other water-insoluble. materials whichv are ,present,-and .a small amount of residual soluble salt dissolved. in water trapped in. the. coagulum.

'l-For some types of emulsions, thiscoagulum may be useddirectly without'further washing. for instance, in the, preparation of emulsions; for the ;.-coating ofpaperor. other permeable supports. For .Ithe; preparationof .emulsionslfor ,lthe coating. of ifilmorglass: plates, however,1it may be. desirable .,.to reduce further the content .of solubletsalts -.which may be present. .This may be ,done. by any one. of:severa1. methods. I. For. instance,.the co- =ag\ulum may be; rinsed by.treatment withcool water -preferably-with the pH .adjustedto the rrange. of the. isoelectricpoint of thegelatin derivative. The: number of. rinses which. are neces- -sary to reduce the saltcontent of thecoagulum to the desiredpoi-nt may be determined by experi- -ment,-.but usually..is.within. the range of. 1 to 4 As .an alternative washing water at an elevated temperature using wateriand ea small amountv of. alkaline material, .such. as sodium hydroxide or-ammonium hydroxide so that .the:pI-I of the mixture remains of. the order ofi6 -or higher. raised to approximately 40. C.,v andthe .whole is .stirred for a few minutes toeifect redispersion of -.the.'coagulum. The gelatin derivative and silver The temperature of .the solution is grains may them again be precipitated by .the

. addition of an appropriate acid solution to reduce thevpH' of the solution to the coagulation point coagulation point of the derivative.

xtion of asssuitable baseesuch assammonium hydroxide, to raise the pI-I of the solution to the The coagulum will settle, and the separation from the supernatant liquoris carried out as before.

The coagulum which has been washed by any onexofithe'.three' general methods outlined above may then be.redispersed to form photographic emulsions suitable for the subsequent finishing .10

and coatingoperations by treating with the re .quired quantity of water, normal gelatin and base .to raisethe pH of the whole to the range of 5 or 6 or greater. If desired gelatin derivative instead of normalgelatinmay beemployed as the vehicle in the formation of the photographic emulsion.

The'mixtureof coagulum, watenzggelatinand any other desired reagents is stirred at approximately 4:0 C. for a time" suiTicient to: effect aicomplete redispersal of .the coagulum, usually requiring ten to twenty-minutes. .The emulsionobtained has properties comparable to those of the best types of washed" emulsions and'responds readily to chemical sensitizationmr .iafter ripening .and optical sensitization ofthe types employed upon photographic emulsions. generally. The. emulsions. prepared iniaccordance with our invention may be coated onto a support, using the techniques which arestandard in the emulsion coating art.

An alternative method for the use;of the gelatin derivative in the preparationof silver-halide dispersions affordsequal advantage. The aromatic isocyanate and sulfonyl ,chloride derivatives of gelatin are especially adapted for, preparing silverhalideidispersions by this method. ."In this methodthe. silver-halide ,dispersionis prepared in normal gelatin solution according to standard techniques? for thepreparation of the silver halide and ripening. which may beJdesiredis carried out.

At this pointythe.pI-I of;1the.mass1.is adjusted to ..a.va1ue.of approximatelyS to11 or.more Withfa suitablebase, such as'sodium. hydroxide or ammonium hydroxide, and the reagent for formation .of .the gelatin derivative is added in suitable physical'i form, .usua'lly.disso1ved. in a solvent. .a. suitable time-has. ela sed. for the. reaction between. the reagent and the gelatin (five minutes is. ordinarily adequate) the pl-Iofthe mass may After .thenbe lowered. to the. coagulation region. .After the formation ofithecoag-ulumby the reduction of the pH, the silver-halide gelatin derivative coagulum. mayv be washed toremove insoluble salts by. either offlthe two .methcids. outlined above.

Theluse.of..gelatin.derivatives by either of the twomethods outlined ,above. presents considerable. advantage over other techniques described in.- the' literature for Washing of emulsions by coagulationmethods. "It, has been well-known for yearsthat. gelatin silverI-halide emulsions could .becoagulated by. the addition of'sufficient quantitiesof. suitable organic solvents, such as meth- ...anol ethanol, or acetone. .Such .a procedure,

however, hasproved to be expensive andcumbersome because. of the very. large quantities of :organim solvent which. must be. usedper unit .of emulsion. .Thegelatin coagulates .and becomes insoluble inthe solvent. medium. through the well-known-effectoforganic solvents-to coagulate'proteins generally.

In the preparation of gelatin silver-halide emulsions bythe .use; of gelatinderivatives the physical characteristics of water-solubility as .relatedato pI-I-are of vital importance. .found that. gelatin derivativespreparedby re- .acting standard photographic gelatins under .conditionsof elevated; pH witharomatic 'sulfonyl We have electrolytes.

soluble halide, such as potassium bromide.

addition to this salt it is common practice that ably one which is water miscible.

diketones posses the characteristic of water solubility at pH values above approximately 4.5 and below approximately 4.0, providing, of course,

salts are absent. In the pH range of approximately 4 to 4.5 these derivatives are relatively water-insoluble. The pH range of insolubility in water is broadened by the presence of neutral For instance, the derivative obtained by reacting benzene sulfonyl chloride with gelatin is insoluble in aqueous solutions over a pH range extending from 3 to 4.5 if a concentration of potassium nitrate of at least 0.1 normal is present. It may be noted, therefore, that in electrolyte is usually present in the form of potassiurn nitrate resulting from the double decomposition reaction between silver nitrate and a In some soluble halide also be present. Under such conditions the adjustment of pH in the first coagulation is not particularly critical. We prefer to add sufficient photographically inert acid to reduce the pH of the emulsion to approximately 3.0 in the presence of such concentrations of salt. It is obvious that when the mother liquor is removed from the coagulum first formed that the largest proportion of soluble salts is also removed. If the coagulum is redispersed in distilled water for washing by raising or by lowering the pH, the second coagulation takes place at a much lower salt concentration and, therefore,

the pH adjustment necessary will be more critical. For this purpose we prefer to operate in the range of 3.8 to 4.2.

For the acid which is to be employed for reducing the pH to the coagulation point, any acid may be employed which will give the desiredpI-I in the mass and which will not deleteriously affect the photographic material. Dilute aqueous sulfuric acid has been found to be particularly useful for this purpose. Neverthelesa other acids are useful and will suggest themselves to those skilled in the art. For instance, acetic acid or phosphoric acid are useful for this purpose if for some reason sulfuric acid is not available.

In the preparation of gelatin derivatives the gelatin in aqueous solution is treated with a solution of the reagent in a suitable solvent, prefer- Some reagents which are useful in this connection are acetone, dioxane, isopropyl alcohol, and the like. of thecornpounds which aresuitable for use in preparing gelatin derivatives useful in the process of our invention are as follows:

S uZfong Z chlorides '1 'chlorosulfonyl 2 hydroxy 4 3 naphthoic acid quinoline-8-sulfonyl chloride ns-Carboxybenzene sulfonyl chloride 2-arnino*5-methylbenzene-sulfonyl chloride Some 1 carboscylz'c acid chlorides Phthalyl chloride p-Nitrobenzoyl chloride Benzoyl chloride Ethyl chlorocarbonate Furoyl chloride Methacrylyl chloride Acid anhydride Phthalic anhydride Benzoic anhydride Succinic anhydride Maleic anhydride Isatoic anhydride Methacrylic anhydride I socyana tes Phenyl isocyanate p-Bromophenyl 'isocyanate p-Chlorophenyl isocyanate p-Tolyl isocyanate p-Nitrophenyl isocyanate a-Naphthyl isocyanate c-Naphthyl isocyanate 1,4-Di7cetones Acetonyl acetone Dimethyl acetonyl acetone Diethyl diacetyl succinate It is apparent that considerable advantage will accrue by the use of this method in that the concentration of silver. halide in the final emulsion may be adjusted to any desired value without reference to the quantities of water, silver halide, and gelatin used in the initial emulsification reaction. Thus, concentrated emulsions may readily be prepared.

We have also found further advantage to result from an application of these techniques.

ration of silver halide as a dry particulate emulsion by a method involving the use of anion soaps has also been revealed.

We have found that the, coagulum prepared by the use of gelatin derivatives is especially suitable for preparation of driedemulsions since it may be obtained in a state nearly free of excess water and, therefore, the drying operation is simplified and rendered less expensive. The coagulum may be obtained in the suitable physical form for the purpose, and the dried emulsion may readily be redispersed in water containing appropriate alkali. In this connectionwe have found that the washed coagulum prepared from the dicarboxylic acid anhydride derivatives, such as phthalic anhydride, is especially useful. These derivatives and their preparation are described and claimed in our application, Serial No. 768,474filed of even date, now Patent No. 2,525,753.

We'have also found that the coagulation operation may be carried out either after the preparation of silver halide to remove watersoluble salts or after a heat treatment with appropriate chemical'sensitizers so that coagulated silver-halidetgrains' 'areiobtainedbefore or after having been brought to optimum photographic speed. These grains maybedriedat eitherv stage .in the cycle without harming their properties when finally made up in the emulsion,

The gelatin derivatives which. may be employed in accordance with ourinvention may be either derivatives of a high viscosity gelatin or, if desired, they maybe derivatives of gelatin which has been hydrolyzed. or reduced in viscosity. Our invention is..not. critical as to the gelatin which is employedin preparing the derivatives to be used in our invention or process including gelatin derivatives generally in .the preparation of washed emulsions by coagulation washing methods.

The following examples-illustrate our invention:

Example 1.-100 g.,of silver 'nitrateweredissolved in 900 cc. of distilled waterat 25 C. 80g. of potassium bromide, 1 g. of potassium. iodide and 25 g. of gelatin weredissolved in another 900 cc. of distilled water. Thetemperature was adjusted to 50 C. and the silvernitrate solution was added to the bromide-gelatin solution over a period of approximately ten minutes with vigorous stirring. The temperature was maintained at 50 C. The mass was then stirred for ten minutes at 50 Qand then cooled t 40 C. The pH was adjusted to. 9.5 by the addition of dilute ammonium hydroxide and 3 cc. of benzene 'su-lfonyl chloride in 40 cc. of dry acetone were added. The "mass-was stirred at 40 C. for five minutes; Sulfuric acid was then added to. ad-

just the emulsion pI-I to 3, and the gelatin derivative which had formed coagulated taking "with itthe silver halide.

The mother. liquor was decanted oh, and the silver halide disper- "sion' waswa'shedtwice with distilled water at -'20' C. There was then added to the mass 150 g. of gelatin in solution in 1500 cc. of distilled water, and the mass was stirred at 40C. Ammonia was added to bring the emulsion pH to 615, and stirring was continued until all of the solid matter was completely dispersed in the "liquid. The resulting emulsion was then processed'and coated as is'usual for photographic emulsions;

EwampZe"2.The use 0] them-carbony benzene suijonyl derivative of gelatin for making silverhalidedispersions: The gelatin derivative was prepared by dissolving 100 g. of gelatin in 900 cc. of distilled waterat 40 C. The solution was then' adjus'ted'ito a pH of with 10%aqueous sodiumhydroxide, and there was then added 10.5 g; vofm-carb'oxybenrene sulfonyl chloride in 50 cc. of drya'cetone over a 2-hour period. The temperature andipI-I was maintained by the ap- =plioation ofzheat'andithe occasional addition of alkali; The pH wasthen adjusted to 7.0, the gelatin. derivative was set by chilling, sliced,

; washed, and: dried.

,TOT'EL solution of 80g; ot'potassium bromide, 1:g:.of=postassiurn iodide, and125 g. of'thegelatin derivative prepared as described in 900 cc. of

,to ltO reduce the pH to .3.0. A coagulum of silver halide and the. gelatin-derivative formed and was allowed. to :settle..out-v whereupon the. liquid was "decanted therefrom. A -volume-of-distilledr water approximately equal to the-original volume was added; thev mass washeated to 35 C. and. the pH was-adjusted while stirring to 6.0-8.0. The coagulum redispersed whereupon. dilutesulfuric acidwas added toigiveva pH of: 3.8-4.2. The coagulum was again acaused to settle to. the :bottomwof the container and the supernatant-liquid was-decanted. 'Ilhe emulsion was made up to the desiredweight bythe addition of water and gelatin. A.uniform.dispersion of the coagulum was-obtained-by stirring at 40 C. and a pH of approximately 7.0.. Afteryappropriate digestion the emulsion.:.showe-d .normal sensitometric .char- .acteristics when .coated ontplates.

Example 8.--Useof the. beneoyl chloride ederinatine of gelatin in making ,silver-halideudispersions: The gelatin derivative was prepared by dissolving ,-l001g of gelatin in.1-1-00. cc. of water vat 40 C., adjustingthepI-I to 53.5-10.0. with 20% sodium hydroxideqand adding 15 g. of benzoyl chloride over a short period (3-20 minutes). The wasmaintained at about 10.0 by occasional addition of alkali. The-reaction was continuedat 40 C. for one hour, the pH=of the mixture, was lowered to 6-7 with dilute sulfuric acid, and the gelatin derivativewa's chill set, sliced; anddried.

There. was mixed together. in. 900 .cc; of distilled ;water'at'50 0., 800 g. of potassium bromide, 1g. of potassiumiodide, and 25g. of the gelatin derivative. A solutionof 100 g. ofsilver nitrate fin-900 cc. of distilled water also at C. was added thereto 'overa period of ten minutes. The temperature "of theresulting emulsion was maintained at 50C.. for another ten-minutes, followingwhich. the emulsion was cooled to 35 C. and a quantity of dilute-sulfuric acidsuflicient :to lower the pH of thesemulsionrto; 3.0 was added. When the coagulum had settled, the supernat'antliqu-id was decanted and thecoagulum was rinsed with. three changes of cold water. There wasthenadded 125g. of gelatin and distilled water to bring the weight of the mass to 5z5 l-bs. The pH' was adjusted to 6-7 with dilute" ammonium hydroxide, and-0.250.5

g. of potassum bromidewas added. The emulsion was finished by heating at C. for 20-40 minutes and was then coated onto glass plates where theemulsion layer was-dried and tested sensitometrically by'exposure in .a sensitometer and development. in. a photographic developer.

The emulsion showed normal .sensi-tometriccharacteristics.

ExampZe 4.-Use. of. the faroyl. chloride de- Tivatine of gelatin in preparing silver-halide dispersions: The gelatinderivative was prepared in a manner similar. tothatof Example Bexcept that only 7.5 g. of alpha-furoyl chloride was used instead of'l5 g.- ofbenzoyl chloride.

The emulsion was prepared as in Example-3 using this furoyl .halide derivativeof gelatin as the peptizing agent. .After the silver-halide grains had been thoroughly washed, the emulsion was cooled to 35, the .pI-I.w,as.lowered..to..3 with dilute sulfuric. acid, the coagulumwas washed, rinsed with three changes .of .cold water and then made up withthe desired quantities of gelatin and water. The-.coagulated silverhalide grains and gelatin derivative. were readily dispersed-at a pH of 6-7; The emulsion was made up with an. aqueous solution of gelatin. The pH was adjustedand potassium bromide was added as described herein. The emulsion was finished by- .heating at..65. C. for...a.- short time a and was coated on film support which was dried and tested photographically. The emulsion showed normal sensitometric characteristics.

Example 5.--The use of the phthalz'c anhydride derivative of gelatin in making silver-halide dispersz'ons: The gelatin derivative was prepared by the method described in Example 3 except that g. of phthalic anhydrideiin 35 cc. of dry acetone was used instead of the benzoyl chloride. The silver-halide dispersion was prepared by adding to a solution of80 g. of potassium bromide 1 g. of potassium iodide and g. of the gelatin derivative in 900 cc. of distilled water at 50 C., a solution of 100 g. of silver nitrate in 900 cc. of distilled water at 50 C. all over a period of ten minutes. The temperature of the resulting mass was then maintained at 50 C. for another ten minutes. The silver-halide dispersion was then cooled to C. and dilute surfuric acid was added thereto to bring the pH to 3. The clear supernatant liquid formed upon standing was decanted. There was then added a volume of distilled water approximately equal to the original volume of the emulsion and stirring was continued at 35 C. and a pH of 3-3.5 until the coagulum was dispersed. Dilute ammonium hydroxide was then added to adjust the pH to 3.8-4.2 at which point the gelatin derivative and silver halide settled to the bottom of the container. After decanting the liquid the silverhalide dispersion was made up with gelatin and water and with proper finishing gives an emulsion having normal sensitometric characteristics. Instead of being made up directly into an emulsion the washed coagulum consisting of silver-halide grains and the gelatin derivative may be dried. When it is desirable to use the dried coagulum, it may be employed by soaking in water for a time whereupon it may be readily dispersed at a pH of approximately 6 or 7.

Erample 6.-The use of the maleic anhydride derivative of gelatin as the pcptieer in making silver-halide dispersions: The maleic anhydride derivative of gelatin was prepared in the same manner as described in Example 3 except that 4 g. of maleic anhydride was employed in reacting upon the gelatin instead of the benzoyl chloride. The emulsion was prepared by mixing a solution of 80 g. of potassium bromide, l g. of potassium iodide and 25 got gelatin in 900 cc. of distilled water at 50 C. and adding over a period of ten minutes a solution of 100 g. of silver nitrate in 300' cc. of distilled water at the same temperature. The temperature of the dispersion was maintained at 50 C. for an additional ten minutes, was then cooled to 35 C. and dilute sulfuric acid was added to impart a pH of 3.0. The coagulum settled to the bottom of the container, the supernatant liquid was decanted, and a volume of water equal to the original volume was added. The temperature was adjusted to 35 C. and while stirring the pH was adjusted to 6-8 with dilute ammonium hydroxide. The coagulum was thus redispersed in water. Dilute sulfuric acid was added to impart a pH of 3.8-4.2 whereupon the coagulum again settled. The liquid was decanted, and the silver-halide dispersion was mixed with an aqueous gelatin. The resulting emulsion was coated on film base and tested for photographic characteristics. Normal sensitometric characteristics were obtained therefrom on finishing.

Example 7.Use of the phenyl ureido derivative of gelatin in preparing silver-halide dispersions: A silver-halide dispersion was preparedby;

preparing a solution of g. of potassium bromide, 1 g. of potassium iodide and 25 g. of gelatin in 900 cc. of distilled Water at 50 C. There was then added to this solution over a period of ten minutes a solution of g. of silver nitrate in 900 cc. of distilled water, also at 50 C. The resulting dispersion was maintained at 50 C. for ten minutes. The mass was then cooled to 40 C., the pH of the emulsion was adjusted to approximately 9.0 with dilute ammonium hydroxide, and a solution of 2.5 g. of phenyl isocyanate in 30 cc. of dry acetone was added thereto and stirred in for approximately five minutes while maintaining the pH of the emulsion at about 9.5 by periodical additions of dilute ammonium hydroxide. The pH of the emulsion was then adjusted to 3 by the addition of dilute sulfuric acid. The silver halide and gelatin derivative were thereby caused to settle, and the clear, supernatant liquid was decanted. The coagulum was dispersed inan equal volume of distilled water at 40 C. at a pH of approximately 3.0. The pH was then raised to 3.8-4.2 with dilute ammonium hydroxide whereupon the coagulum again formed and settled and the liquid was decanted. A photographic emulsion was made up using this coagulum with the required amounts of gelatin and water as described in the preceding examples. The photographic emulsion upon testing was found to have normal sensitometric characteristics on finishing.

If desired, instead of preparing the silver halide in the gelatin solution, the gelatin derivative prepared may be employed as the dispersing colloid during the silver halide precipitation. The coagulation procedures in both cases are identical.

Example 8.Use of the p-tolyl ureido derivatire of gelatin in making silver-halide dispersions: A gelatin derivative was prepared-in the same manner as described in Example 3 except that 7.5 g. of p-tolyl isocyanate was employed instead of the benzoyl chloride. The silver halide was prepared in a solution of this gelatin derivative in the same manner as described in Example 3. After maintaining the dispersion at 50: C. for

ten minutes, it was cooled to 35 C., and the pH of the emulsion was adjusted to 3.0 with. dilute sulfuric acid. When the coagulum had settled, the clear, supernatant liquid was decanted therefrom. An equal volume of distilled water was added, and the mass was stirred at 35 C. at a pH of 3-3.5 until the coagulum had dispersed. Recoagulation was then effected by adding dilute ammonium hydroxide to impart a pH of 3.8-4.2. The dispersion thus obtained was employed in preparing a photographic emulsion by mixing gelatin and water therewith as described in Example 3 whereupon the emulsion was tested as described there. It was found that normal sensitometric characteristics were present in this emulsion.

Example 9.-A gelatin derivative was prepared in the same manner as in Example 3 except that diethyl diacetyl succinate was added before rais-' ing the pH to 10, and the reaction time was six hours, the diethyl diacetyl succinate being employed instead of benzoyl chloride. The silverhalide dispersion was prepared as described in that example using the diethyl succinate derivative of gelatin as the dispersing colloid. After the temperature of the dispersion had been maintained at 50 C. for ten minutes, the emulsion was cooled to 35 C. and dilute sulfuric acid was added to impart a pH of. 3. Whenthe 00- Ill agui uiirnaa settled, the clear, supernatant liquid wa's-decanted and the coagulum was washed with three changes of cold-water, taking care to break upthe large masses to perrnit thorough washing. Thiscoagulum wasthen employed to prepare a photographic emulsionby mixing therewith the desired: quantities of gelatinin water and stirring until-the whole was dispersed with'the pH raised to"? with dilute ammonium hydroxide. The emulsion was tested as described in the preceding examples and showed satisfactory sensitometric data as to speed, fog, and contrast, following suitabledigestion.

It is-to beunderstood'that in'all of th procedures described herein for the preparation of silver-halidedispersionsthat the preparation of the-silver halide is carried-out under darkened conditions so-as to preserve the l-ight-sensitiveness of thatm-aterial.

We claim:

1. -I n amethod of preparing washed silver halide-dispersions, thesteps which comprise mixtogether a-water-soluble silver salt and a water-soluble halide salt in an aqueous solution of 'a gelatin-derivative whose presence renders the-dispersioncoagulable at 'a pH within the range of pH- 3-4:.5, which derivative is selected from the group consisting of the aromatic sulfonyl chloride-derivatives of gelatin, the carboxylic acid chloride derivatives of gelatin, the carboxylic acid anhydride derivatives of gelatin. the aromatic isocyanate derivatives of gelatin and the- 154 diketone derivatives of gelatin, coagulatlngthe dispersion of silver halide in gelatin derivative thusformed by adjusting to said pH, whereby silver halide-gelatin derivative grains are formed, and subsequently separating the silver halide-gelatin derivative grains thus formed. from i the liquid portion of the mass.

In a'method of preparing washed silver halide dispersions, the steps which comprise n'i'ixing-together a water-soluble silver salt and a water-soluble'halide salt in an aqueous solution of-"an'aromatic sulfonyl chloride derivative of gelatinwhose presence renders the dispersion coagulable' at a pll within the range of pH 3-4.5, coagulatingthe dispersion of silver halide in gelatin derivative thusformed'by adjusting to said-"pH wherebysilver halide-gelatin derivative grains are formed and subsequently separating the" silver halide-gelatin derivative grains thus formed fromthe liquid portion of the mass.

3-. In-a method of preparing'w-ashed silver halide dispersions, the steps which comprise mixingtogether' a-water-soluble silversalt and a'water scluble' halide salt in'an aqueous solution ofacarboxylic acid chloride derivative of gelatin whose presence renders the dispersion coagulable at a pH within the range of pH 3,-4.5, coagulating the dispersion of-silver halide in gelatin derivative thus formed by adjusting to said pH whereby silver halide-gelatin derivative grains; are formed and subsequently separating the silver'halide-gelatin derivative 1 grains thus formed from the liquid portion of the mass.

In a method'of preparingwash'ed silver halide dispersions, the steps which comprise mixingiutogether a water-soluble silver salt and a water-soluble halide salt in an aqueous solution or a carboxylic acid anhydride derivative of; gelatin whose presence renders the dispersion coagulable at a pH within the range or pH 3-4151, coagulating-the dispersion of silver halide in gelatin derivative thus formed by adjusting to said pH whereby silver halide-gelatin deriva- 12 tive grains are formed andsubsequently-separating the silver halide-gelatin derivative grains thus formed from the liquid portion of the. mass.

5. In a method of preparing washed .silver halide dispersions, the steps which comprise mixing together a water-soluble silver salt and a water-soluble halide salt in an aqueous solu tion of an aromatic isocyanate derivative. of gelatin whose presence renders the dispersion coagulable at a pH within the range of pH -4.5, coagulating the dispersion of silver halide in gelatin derivative thus formed by adjusting to said pH whereby silver halide-gelatin derivative grains are formed and subsequently separating: the silver halide-gelatin derivative grains thusformed from'the liquid portion of the mass.

6. In a method of preparing washed silver halide dispersions, the steps which comprise mixing together a water-soluble silver salt and a water-soluble halide salt in an aqueous.-solu-' tion of 1,4 diketone derivative of gelatin whosepresence renders the dispersion coagulable ata pH within the range of pH 3-4.5, coagulating the dispersion of silver halide in gelatin'derivative thus formed by adjusting to said pH whereby silver halide-gelatin derivative grains are formed and subsequently separating the silver halide-gelatin derivative grains thus formed from the liquid portion of the mass.

'7. In a method of preparing washed silver halide dispersions, the steps which comprise Il'llXiIlg together a water-soluble silver salt and a water-soluble halide salt in an aqueous solution of the benzene sulfonyl chloride derivative of gelatin whose presence renders the dispersion coagulable at a pH within the range of pH 3-4.5, coagulating the dispersion of silver halide in gelatin derivative thus formed by adjusting to said pH whereby silver halide-gelatin derivative grains are formed and subsequently separating the silver halide-gelatin derivative grains thus formed from the liquid portion of the mass.

8, In a method of preparing washed silver halide dispersions, the steps which comprise mixing together a water-soluble silver salt and a water-soluble halide salt in an aqueous solution of the benzoyl chloride derivative of gelatin whose presence renders the dispersion coagulable at a pH within the range of pH 3-4.5, coagulating the dispersion of silver halide in gelatin derivative thus formed by adjusting to said pH whereby silver halide-gelatin derivative grains are formed and subsequently separating the silver halide-gelatin derivative grains thus formed from the liquid portion of the'mass.

9. In a method of preparing washed silver halide dispersions, the steps which comprise mixing together a water-soluble silver salt and a water-soluble halide salt in an aqueous solution of the phthalic anhydride derivative of gelatin whose'presence renders the dispersion coagulable at a pH within the range of pH 3-4.5, coagulating the dispersion of silver halide in gelatin derivative thus formed by adjusting to said pH whereby silver halide-gelatin derivative grains are formed and subsequently separating thesilver halide-gelatin derivative grains thus formed from the liquid portion of the mass.

10. In a method of preparing washed silver halide dispersions, the steps which comprise mixing together a water-soluble silver salt and a water-soluble halide salt in an aqueous solution of, thetolyl isocyanate derivative of gelatin whose. presence renders the dispersion coagulable at a pH within the range of pH 3-4.5, cow

13 agulating the dispersion of silver halide in gelatin derivative thus formed by adjusting to said pH wherebysilver halide-gelatin derivative grains are formed and subsequently separating the silver halide-gelatin derivative grains thus formed from the liquid portion of the mass.

11. A method of preparing a gelatino-silver halide emulsion which comprises preparing a silver halide dispersion by mixing together a water-soluble silver salt and a Water-soluble halide in an aqueous solution of a phthalicanhydride derivative of gelatin whose presence renders the dispersion obtained coagulable at a pH within the range of pH 3-4.5, coagulating the dispersion of silver halide in gelatin derivative by adjusting to said pH whereby silver halide-gelatin, derivative grains are formed, separating the thus-formed silver halide-gelatin derivative grains from the liquid portion of the mass and subsequently mixing those grains with gelatin and water as a protective colloid for the silver halide whereby a photosensitive emulsion is formed.

12. In a method of preparing washed silver halide dispersions, the steps which comprise mixing together a water-soluble silver salt and a water-soluble halide salt in an aqueous solution of a gelatin derivative whose presence renders the dispersion coagulable at a pH within the range of pH 3-4.5, which derivative is selected from the group consisting of the aromatic sulfonyl chloride derivatives of gelatin, the carboxylic acid chloride derivatives of gelatin, the carboxylic acid anhydride derivatives of gelatin, the aromatic isocyanate derivatives of gelatin and the 1,4 diketone derivatives of gelatin, coagulating the dispersion of silver halide in gelatin derivative by adjusting to said pH whereby silver halide-gelatin derivative grains are formed, separating those grains from the liquid portion of the mass, redispersing the grains thus obtained in water at an elevated pH followed by adjusting the pH of the solution to that which will coagulate the silver halide dispersion which pH is within the range of 3-4.5, and separating the thus-formed silver halidegelatin derivative grains from the liquid portion of the mass.

13. In a method of preparing washed silver halide dispersions, the steps which comprise mixing together a water-soluble halide salt and a water-soluble silver salt in an aqueous solution of a phthalic anhydride derivative of gelatin, whose presence renders the dispersion thus obtained coagulable at a pH within the range of pH 3-4.5, coagulating the dispersion of silver halide in gelatin derivative by adjusting to said pH whereby silver halide-gelatin derivative grains are formed, separating those grains from the liquid portion of the mass, redispersing those grains in water at an elevated pH followed by adjusting the solution to a pH which will coagulate the silver halide dispersion. which pH is within the range of 3-4.5 and separating the thus-formed silver halide-gelatin derivative grains from the liquid portion of the mass.

14. A method of preparing a gelatino-silver halide emulsion which comprises pre aring a silver halide dispersion by mixing together a water-soluble silver salt and a water-soluble halide in an aqueous solution of a gelatin derivative whose presence renders the dispersion coagulable at a pH within the range or pH 3-4.5,

derivatives of gelatin and the 1,4 diketone derivatives of gelatin, coagulating the dispersion of silver halide-gelatin derivative by adjustin to said pH whereby silver halide-gelatin derivative grains are formed, separating the thus-formed grains from the liquid portion or" the mass and subsequently mixing those grains with water and gelatin as a protective colloid for the silver halide whereby a photosensitive emulsion is formed.

15. In a method of preparing washed. silver halide dispersions, the steps which comprise mixing together a water-soluble silver salt and a water-soluble halide salt in an aqueous solution of a maleic anhydride derivative of gelatin Whose presence renders the dispersion coagulable at a pH within the range of 3-4.5, coagulating the dispersion of silver halide in gelatin derivative thus formed by adjustin to said pH whereby silver halide-gelatin derivative grains are formed, and subsequently separating the thus-formed grains from the liquid portion of the mass.

16. A method of preparing a washed gelatinosilver halide photographic emulsion which comprises preparing a silver halide dispersion by mixing together a water-soluble silver salt and a water-soluble halide in an aqueous solution of a gelatin derivative whose presence renders the dispersion coagulable at a pH within the range of pH 3-4.5, which derivative is selected from the group consisting of the aromatic sulfonyl chloride derivatives of gelatin, the carboxylic acid chloride derivatives of gelatin, the carboxylic acid anhydride derivatives of gelatin, the aromatic isocyanate derivatives of gelatin and the 1,4 diketone derivatives of gelatin coagulating the dispersion of silver halide in gelatin derivative thus formed by adjusting to said pI-I whereby silver halidegelatin derivative grains are formed, separating the thus-formed grains from the liquid portion of the mass, redispersing those grains in water at a dispersing pH therefor, followed by adjusting the pH of the solution to that which will coagulate the silver halide dispersion, said pH being within the range of 3.3-4.2, separating those grains from the liquid portion of the mass and mixing them with water and gelatin as a protective colloid for the silver halide whereby a photosensitive emulsion is formed.

HENRY C. YUTZY. GORDON F. FRAME.

REFERENCES CITED The following references are of record in the 

1. IN A METHOD OF PREPARING WASHED SILVER HALIDE DISPERSIONS, THE STEPS WHICH COMPRISES MIXING TOGETHER A WATER-SOLUBLE SILVER SALT AND A WATER-SOLUBLE HALIDE SALT IN AN AQUEOUS SOLUTION OF A GELATIN DERIVATIVE WHOSE PRESENCE RENDERS THE DISPERSION COAGUABLE AT A PH WITHIN THE RANGE OF PH 3-4.5, WHICH DERIVATIVE IS SELECTED FROM THE GROUP CONSISTING OF THE AROMATIC SULFONYL CHLORIDE DERIVATIVES OF GELATIN, THE CARBOXYLIC ACID CHLORIDE DERIVATIVES OF GELATIN, THE CARBOXYLIC ACID ANHYDRIDE DERIVATIVES OF GELATIN. THE AROMATIC ISOCYANATE DERIVATIVES OF GELATIN AND THE 1,4 DIKETONE DERIVATIVES OF GELATIN, COAGULATING THE DISPERSION OF SILVER HALIDE IN GELATIN DERIVATIVE THUS FORMED BY ADJUSTING TO SAID PH. WHEREBY SILVER HALIDE-GELATIN DERIVATIVE GRAINS ARE FORMED, AND SUBSEQUENTLY SEPARATING THE SILVER HALIDE-GELATIN DERIVATIVE GRAINS THUS FORMED FROM THE LIQUID PORTION OF THE MASS. 